Voltage controlled oscillators (VCOs) are used in phase lock loops (PLLs) to generate clocks having particular frequencies. PLLs are generally considered clock multipliers. For example, an input reference clock having a frequency of 10Mhz can be multiplied by the PLL to yield an output clock signal having a frequency of 100Mhz. Ideally, this clock multiplication would result in an output clock which is in perfect phase/frequency with the reference clock. However, a condition generally referred to as VCO "runaway" is a condition that can occur when the output of the VCO is running at such high speed that the PLL (typically the counters in the PLL) are not capable of operating at a sufficient speed to keep up with the VCO output clock signal. This condition can (and does) occur during power up and initial lock acquisition.
FIG. 1 illustrates a phase lock loop circuit 10 illustrating a conventional approach. The circuit 10 generally comprises a phase frequency detector (PFD) 12, a loop filter 14, a voltage controlled oscillator 16 and a divider 18. The VCO 16 presents a signal (i.e., VCO.sub.-- CLK) to the divider 18. The divider 18 presents a feedback signal to the PFD 12. The PFD 12 also receives a reference clock (i.e., CLK). The difference in frequency between the reference clock CLK and the feedback signal is used to generate two control signals (i.e., PumpUP and PumpDN) that are presented to the loop filter 14. The loop filter 14 presents a voltage control signal to the voltage controlled oscillators in response to the control signals PumpUP and PumpDN. During normal operating conditions, the reference clock CLK is generally synchronized with the feedback signal. Such a synchronization is shown by the block 20.
An example of the runaway condition can be described using an example of an 800Mhz PLL 10 having a divider implementing divide-by-10 feedback counters and an 80Mhz reference clock CLK. When the PLL 10 is initially powered on, the control nodes (i.e., the input) of the VCO 16 are set to some arbitrary high voltage. In the example of a wide frequency range VCO 16, oscillations beyond a normal operating range could result. If the feedback counters of the divider 18 are designed to run at 800Mhz or below (either due to current consumption or structural limitations) the output of the feedback counter 18 may be unpredictable. The PLL 10 is likely to generate an output signal having a low frequency related to the underlying noise factors (perhaps in the 10Mhz range). Since the reference clock CLK presented to the PFD 12 is set to 80Mhz and the feedback signal is now oscillating at only 10Mhz, the PFD 12 will generate predominately PumpUP control signals to the loop filter 14. As a result, a further increase in the control nodes to the VCO 16 will occur, which results in the VCO 16 presenting the output clock signal VCO.sub.-- CLK running at an even faster frequency.
In addition, the runaway condition can occur in the VCO 16 when the output clock signal VCO.sub.-- CLK has a frequency that increases to the point where the feedback counters of the divider 18 fail. Alternately, in a case where the feedback from the VCO 16 is derived outside of the chip, a high load can "kill" the frequency of the feedback signal presented to the PFD, which causes a sequence of PumpUP control signals to be presented to the VCO 16.
(The ultimate effects of a VCO runaway condition are dramatic. When implemented in a typical PLL system (such as that shown in FIG. 1), a "lock-up" condition can occur which requires a hard reset to allow the VCO 16 to resume normal operation).
One conventional approach that may be used to reduce the effect of a VCO runaway condition may be to built a feedback counter (and the associated counters and logic) that can always keep up with the operating frequency of the VCO 16. However, to prevent the runaway condition, such logic and counters can be required to run a very fast speeds, which can draw high amount of current. Such a high speed, high current device may be impossible to implement if the VCO 16 is a CML derivative and the counters are implemented in CMOS technology. Additionally, such a solution continues to emphasize these drawbacks as operating frequencies continue to increase.
Another conventional approach is to place a voltage clamp on the control node (i.e., the input) of the VCO 16 that can track the process corners of the particular technology in which the PLL 10 is implemented. However, such as voltage clamp that can function over a wide variety of process corners can be difficult to design. If the clamp does not track the process corners adequately, a lock-up condition may still occur. As the overall frequency of oscillation of the PLL 10 increases, the difficulty in designing a voltage clamp that tracks a process corners increase.